1. Field of the Invention
The present invention generally relates to a transmission device and a communication system, and more particularly to an optical communication system employing a synchronous digital hierarchy and a transmission device suitable for such an optical communication system.
An optical communication network has been practically used as means for providing broadband services in which a variety of data on telephone, facsimile, images and so on is integrated. The user/network interface in the optical communication network has been internationally standardized, and is known as a Synchronous Digital Hierarchy (SDH), as defined in the CCITT recommendations G707, G708 and G709, the disclosure of which is hereby incorporated by reference. A network which conforms to the SDH has been practically used as SONET (Synchronous Optical NETwork) in the North America.
2. Description of the Prior Art
First, a description will be briefly given of the SONET. The SONET is described in, for example, William Stallings, "ISDN and Broadband ISDN, Macmillan Publishing Company, 1992, pp. 546-558.
In the SONET, a multiplexed optical carrier (OC) is transmitted. The transmission device converts the optical signal (carrier) into an electric signal and vice versa. The electric signal is called a synchronous transport signal (STS). The basic bit rate of the SONET is 51.84 Mbps. The optical carrier having the above basic bit rate is expressed as OC-1. Generally, an optical carrier or signal is expressed as OC-N where N (optical carrier level N) is an integer, and a corresponding electric signal is expressed as STS-N (synchronous transport carrier level N). For example, the optical carrier OC-12 is an optical carrier or signal having a bit rate of 622.080 Mbps (=12.times.51.84 Mbps). In the SONET, signals having bit rates which are integer multiples of the basic bit rate. The optical carrier OC-12 is obtained by multiplexing 12 STS-1 signals at the byte level to thereby generate an STS-12 signal and by converting the STS-12 signal into an optical signal. Generally, the multiplexing of STS-N signals employs a byte-level interleave process.
It will be noted that the STS-3 in the SONET corresponds to a synchronous transport module STM-1 in the SDH. Similarly, the STS-12 corresponds to the STM-4.
The signal STS can be obtained by, for example, sequentially multiplexing digital signals having lower bit rates, such as DS-0 (64 Kbps), DS-1 (1.5 Mbps), DS-2 (6.3 Mbps) and DS-3 (45 Mbps).
FIG. 1 is a block diagram showing the outline of a network of the SONET. Electric signals from terminals 1 and 2 are respectively multiplexed by transmission devices 3 and 7, and resultant multiplexed signals are converted into light signals, which are then sent to transmission paths 8 formed of optical fiber cables. Repeaters 4, 5 and 6 are provided in the transmission paths 8. Particularly, the repeater 5 has a function of terminating the optical signals (the above function is called an add/drop function). As shown in FIG. 1, terms "section", "line" and "path" are defined in the SONET. The section corresponds to an optical transmission part between transmission devices, between repeaters or between a transmission device and a repeater. The line corresponds to an optical transmission part between transmission devices, between repeaters or between a transmission device and a repeater, each having the terminating function. The path indicates the end-to-end optical transmission part.
FIG. 2A is a diagram showing the frame format of the signal STS-1. As shown in FIG. 2A, the signal STS-1 consists of 810 octets, and is transferred every 125 .mu.s. The 810 octets consists of nine rows arranged in a matrix formation, each of the rows consisting of 90 octets. In other words, the signal STS-1 has a 9.times.9 matrix formation. The first three columns (three octets.times.nine rows) forms an overhead in which a variety of control information concerning transmissions. The first three rows of the overhead forms a section overhead, and the remaining six rows forms a line overhead. The control information forming the overheads is also referred to as overhead information.
FIG. 2B is a diagram showing the frame format of the signal STS-3. In the SDH, a new format is not created during the hierarchically multiplexing operation. That is, the signal STS-3 can be formed by simply byte-multiplexing the signals STS-1 including the headers thereof without forming a new header specifically directed to the signal STS-3.
FIG. 3A shows the section overhead and the line overhead, and FIG. 3B shows the path overhead. The bytes forming these overheads are well known, and a description thereof will be omitted here.
FIG. 4 is a block diagram of a practical SONET system. Transmission devices 10A, 10B, 10C and 10D, each capable of operating at a highest bit rate, are connected in a dual loop (ring) formation by means of optical fiber cables 11.sub.1 and 11.sub.2. The dual loop formation facilitates to the flexibility and expansibility of constructing the system. As will be described later, reference numbers 20A-20D indicate transmission devices according to the present invention.
A transmission device having a bit rate equal to or lower than that of the highest bit rate can be connected to each of the transmission devices 10A-10D. In the case of FIG. 4, transmission devices 12a, 12b, 12c, 12d, . . . , each having a bit rate lower than that of the transmission device 10A are connected to the transmission device 10A. The transmission device 10A multiplexes signals sent by the transmission devices 12a-12d and receives via optical fiber cables 13a, 13b, 13c and 13d, and sends a multiplexed optical signal to either the transmission device 10B and 10D or both thereof. In FIG. 4, for the convenience sake, one of two input/output sides of each of the transmission devices 10A-10D is called an east side, and the other side is called a west side. For example, the transmission device 10D is located at the east side of the transmission device 10A, and the transmission device 10B is located at the west side thereof.
Although not shown in FIG. 4, transmission devices having a bit rate lower than those of the transmission devices 12a-12d can be connected thereto by optical fiber cables or electrically conductive cables. Signals from terminals such as telephone sets, facsimile machines and personal computers are multiplexed in accordance with a given hierarchy, and multiplexed optical signals are transferred via the transmission devices 10A-10D. In practice, the transmission devices 10B and 10D, for example, may be regenerators (repeater devices).
As shown in FIG. 5, a network can be constructed by combining a plurality of loops. In FIG. 5, transmission devices 10E and 10F form a loop together with the transmission devices 10A and 10D.
The hierarchy employed when the transmission devices 10A-10D transmit OC-48 light signals is as shown in FIG. 6. Each of the transmission devices 10A-10D transmits an OC-48 light signal, which corresponds to an STS-48 electric signal having 48 multiplexed channels. The OC-48 light signal can be produced by, for example, multiplexing four OC-12 light signals from the transmission device 12a or the like. Each OC-12 light signal can be produced by multiplexing four OC-3 light signals from a transmission device (not shown in FIG. 1) having a lower bit rate.
FIG. 7 shows a hierarchy employed when the transmission devices 10A-10D transmit OC-192 light signals. The OC-192 light signal can be produced by multiplexing four OC-48 signals, which can be produced by multiplexing four OC-12 signals, which can be produced by multiplexing four OC-3 signals. The hierarchy shown in FIG. 7 enables a frame structure called a concatenated STS-N signal (expressed as STS-Mc). In FIG. 7, a STS signal having three channels and corresponding to the OC-3 light signal, that is, an STS-3C signal is processed as one signal and is subjected to a given process such as a multiplexing process. An STS signal having 12 channels and corresponding to the OC-12 light signal is processed as one signal and is subjected to the multiplexing process and so on.
Two transmission methods or protocols applied to the dual ring formation connecting the transmission devices 10A-10D are known. One transmission method is a uni-directional path switched ring method (hereinafter referred to as a UPSR method), and the other transmission method is a bi-directional line switched ring method (hereinafter, BLSR method). In the UPSR method, each of the transmission devices 10a-10d sends an identical light signal to both the east side and the west side.
As shown in FIG. 8, in the UPSR method, the transmission device 10C, for example, sends the identical light signals to the transmission devices 10B and 10D. In this case, one of the two direction forms the working system, and the other direction forms the protection or spare system. The UPSR system is suitable for a case where it is required to distribute the identical signals to nodes (transmission devices). The transmission device 10A receives the identical light signals, and selects one of these signals. The selected light signal is sent to, for example, a transmission device operating at a bit rate lower than that of the transmission device. The above operation is carried out in a normal state. If a fault such as a braking of the optical fiber cable provided between the transmission devices 10A and 10B occurs, an alarm indication signal (AIS) is sent to the following transmission device, and the path is switched to the direction in which there is no alarm indication signal. In the above case, the transmission device 10A selects the light signal coming from the transmission device 10D. Since the identical light signals are sent in the two direction, the number of available channels in each of the optical fiber cables 11.sub.1 and 11.sub.2 coincides with the transmission capacity thereof (for example, 48 channels for OC-48).
In the BLSR system, the light signal is sent in only one direction irrespective of whether a fault occurs. For example, the channels can be used for any of transmissions carried out between the transmission devices 10A and 10C, transmissions carried out between the transmission devices 10C and 10B and transmissions carried out between the transmission devices 10C and 10D. Hence, the BLSR method has a transmission capacity per channel which is equal to three times that obtained in the UPSR system. However, in practice, it is required to provide protection channels, the transmission capacity per channel in the BLSR is not as large as the above, and a redundant configuration is employed. For example, in the OC-48 signal, 24 channels are used as working channels, and the remaining 24 channels are used as protection channels. In this case, a transmission capacity equal to 72 channels (24.times.3 channels) is available. In this case, the transmission capacity in the BLSR system is 1.5 times as large as that obtained in the UPSR method.
When a fault occurs in the BLSR system, the following procedure is carried out. Referring to FIG. 9A, data is transferred from the transmission device 10C to the transmission device 10A via the transmission device 10D. It will now be assumed that a fault occurs in the optical fiber cable 11.sub.1 between the transmission devices 10A and 10D. When the transmission device 10A receives the aforementioned alarm indication signal indicative of occurrence of a fault, the transmission device 10A recognizes the occurrence of a fault, and informs the transmission devices 10B, 10C and 10D of the occurrence of a fault by means of given information, which is an automatic protection switch (APS) including K1 and K2 bytes shown in FIG. 3A.
Upon receiving the APS information, as shown in FIG. 9B, the transmission device 10D makes a loop-back formation for the working channels in the optical fiber cable 11.sub.1 via which the light signal from the transmission device 10C is received so that the above work channels are coupled to the protection channels of the optical fiber cable 11.sub.2 via which the light signal is sent to the transmission device 10C. Further, the transmission devices 10C and 10B form through-lines from the transmission device 10D to the transmission device 10A. Further, the transmission device which detects the fault makes a loop-back formation for the optical fiber cable 11.sub.2 extending from the transmission device 10B so that the protection channels of the cable 11.sub.2 are coupled to the working channels of the optical fiber cable 11.sub.1 extending from the transmission device 10D. Hence, the light signal can be recognized as if it is transmitted over the optical fiber cable 11.sub.1 having a fault. As described above, the BLSR method can efficiently utilize the channels to realize node-to-node communications.
It can be seen from the above description that the channel allocation employed in the UPSR method is quite different from that employed in the BLSR method. Thus, the two methods require respective fault recovery protocols.
Conventionally, the optical communication system is designed and constructed in conformity with either the UPSR method or the BLSR method. Hence, the transmission devices 10A-10F has the channel allocating function based on either the UPSR method or the BLSR method. That is, the transmission devices based on the UPSR method requires the configuration of selecting one of the two light signals, that is, the path switch. The transmission devices based on the BLSR method requires the configuration of enabling the loop-back formation.
However, the above prior art has the following disadvantages. The prior art does not flexibly satisfy various user's demands. For example, if it is required to change a transmission path including transmission devices based on the UPSR method to that based on the BLSR method, all the transmission devices should be exchanged by those based on the BLSR method. This is troublesome and expensive. In other words, the prior art cannot provide a system in which the UPSR method and the BLSR method coexist.